1.Field of the Invention
The present invention relates to a servo apparatus for an optical disk player.
2.Description of tbe Related Art
Disk players of, for example, optical type which play information-recorded disks, such as a video disk and digital audio disk (hereinafter simply referred to as disk) inevitably require a focus servo apparatus for converging a light beam on the information-recorded surface of a disk to form an information reading light spot and a tracking servo apparatus for permitting the information reading light spot to accurately track record tracks on the disk.
As the focus servo apparatus, there is, for example, a known apparatus employing a so-called astigmatism method, which uses a cylindrical lens to converge light beams as a single light beam in a horizontal direction on one of two points separated on the light path of the light beams and converge the light beams as a single vertical beam on the other point, and generates a focus error signal based on the foursome output of a four-split light detector disposed in the middle of the mentioned two points.
As the tracking servo apparatus, there has been known, for example, an apparatus employing a so-called 3-beam method which arranges three beams, namely, an information reading main beam and two sub-beams for detection of a tracking error, located on the respective sides of the main beam, in such a manner that lines connecting the optical axes of the three beams have predetermined offset angles to the direction of a tangent line to the tracks, and generates an error signal based on the difference between the amounts of light of two sub-beams which have passed the information-recorded surface of a disk.
The loop gains of the servo loops of these servo apparatuses may vary due to a variation in reflection factor, a variation in power of a light beam caused by a stain or the like on an objective lens or a variation in sensitivity of a photosensor in a pickup. This variation in loop gain reduces the follow-up performance with respect to the focus error and tracking error, making it difficult to provide a stable servo operation.
In this respect, an automatic gain control is executed for each disk to acquire the optimal loop gain at the time the disk is played. For instance, in automatically controlling the gain of the focus servo loop of, for example, a video disk player, the loop gain is set in advance lower than the one for the normal playing, and the servo is led in to close the loop before controlling the gain. This control method cannot automatically control the loop gain when the focus servo loop cannot be closed due to a variation in reflection factor of the disk or a stain or the like on the objective lens.
Since a variation in tracking error signal is too large to close the servo loop in the tracking servo apparatus, the p-p value of a tracking error signal (the difference between positive and negative peaks) in an open status is detected and the gain control is executed in accordance with this p-p value. As this method performs the gain control with the servo loop in the open status, it is easily influenced by the eccentricity of the disk and is therefore apt to cause a variation in the control value.